A typical thin film bulk acoustic resonator (FBAR) is a tri-layer device that includes a bottom electrode and a top electrode made, for example, from molybdenum. Piezoelectric material, e.g. aluminum nitride (AlN), interposes the two electrodes. This device is deposited over a depression or “swimming pool” made into a substrate, e.g. silicon, where this depression is filled with a sacrificial material, as disclosed by Ruby, et al. in “SBAR Structures and Method of Fabrication of SBAR/FBAR Film Processing Techniques for the Manufacturing of SBAR/FBAR Filters”, U.S. Pat. No. 6,060,818, issued 9 May 2000, assigned to Agilent Technologies. When the sacrificial material is removed, a “free-standing membrane” is created where the edges of the resonator are anchored around the perimeter to the silicon substrate. FIG. 1 shows a cleaved portion of a prior art acoustic resonator over the pool and anchored at the edge of the pool and then connected to a pad.
The active area of this resonator is defined by the overlap between the top and bottom electrodes. Typically, the bottom electrode spans the entire swimming pool to maximize mechanical robustness, as disclosed by Ruby, et al. in “Cavity spanning Bottom Electrode of a Substrate-Mounted Bulk Acoustic Resonator”, U.S. Pat. No. 6,384,697, issued 7 May 2002, assigned to Agilent Technologies. The top electrode is pulled inside of the swimming pool (where possible) by an amount that maximizes the Q of the system as taught in Ruby, et al. in “Bulk Acoustic Perimeter Reflection System”, U.S. Pat. No. 6,424,237, issued 23 Jul. 2002, assigned to Agilent Technologies.
The resonator may also include a mass-loading layer substantially covering the total area of the top electrode. This layer lowers the resonant frequency of the resonator. This layer allows for differentiation by frequency for filters using ladder, half-ladder or lattice type topologies. A half-ladder filter is made of cascaded series and shunt resonators. Mass loading lowers the frequencies of the shunts relative to the series resonators.
For these filters to be successful, the quality factor or Q of each of the resonators comprising the filter must be very high. The Q is the amount of radio frequency (rf) energy stored in the resonator divided by the amount of energy lost to the resonator by Various means. If there is no loss of energy in the resonator, the Q would be infinite. The actual energy stored in the resonator at this frequency is in the form of mechanical motion. There is, however, always some loss. One loss mechanism is thermal acoustic loss where mechanical energy that is converted into heat, e.g. energy lost to the system as heat, is not readily converted back into rf energy.
Energy loss at the edges comes from two sources. First, acoustic energy converted into in the form of lateral modes can leak out from the sides of the resonator and escapes into the substrate. Very little of this energy is recovered by the resonator. Second, there is typically poor delineation and quality of the films at the edges due to these edges being exposed to various dry and wet chemical processes. Lateral modes will “sample” these rough edges and lose energy through scattering off the rough edges and through acoustic migration of atoms at the edges. Thus, it is important to minimize the interaction of lateral modes with the edges of the resonators.